This invention relates to spread-spectrum communications and more particularly to a system and method for a high capacity spread-spectrum communications channel.
Referring to FIG. 1, message data, d(t), are processed by spread-spectrum data modulator 51, using a message-chip-code signal, g1(t), to generate a spread-spectrum data signal. The data spread-spectrum signal is processed by transmitter 52 using a carrier signal at a carrier frequency, fo, and transmitted over communications channel 53.
At a receiver, a spread-spectrum demodulator 54 despreads the received spread-spectrum signal, and the message data are recovered by synchronous data demodulator 60 as received data. The synchronous data demodulator 60 uses a reference signal for synchronously demodulating the despread spread-spectrum signal. The square-law device 55, bandpass filter 56 and frequency divider 57 are well known in the art for generating a reference signal from a received modulated data signal. A Costas Loop or other reference signal generating circuit is also adequate.
The spread-spectrum system of FIG. 1 is limited to a single communications channel, and would not work well for communicating high capacity information using spread-spectrum in a fading environment. Consider the T1, Ethernet, and T3 networks, by way of example, and assume the data modulator 51 spread-spectrum processes the message data, d(t), with a chip rate of 25 megachips per second. For the T1 network which communicates data at up to 1.544 megabits per second, the spread-spectrum system of FIG. 1 has a processing gain of approximately 17. For the Ethernet, which communicates data at up to 10 megabits per second, the spread-spectrum system of FIG. 1 has a processing gain of 2.5. For the T3 network, which communicates data at up to 43 megabits per second, the spread-spectrum system of FIG. 1 has a processing gain of approximately 0.6. Thus, for a particular spread-spectrum channel, data communicated at a higher rate results in a lower processing gain. The lower processing gain can result in channel degradation and loss of the advantages of spread-spectrum. For example: resistance to fading caused by multipath and inability to share the spectrum with other spread spectrum systems such as a PCN system.
Information may be transmitted through the spread-spectrum channel by using M-ary modulation schemes, such as quadrature phase-shift-keying (QPSK) modulation or 16-ary amplitude and phase modulation, for increasing the data rate through the channel. The M-ary modulation schemes are modulated with spread spectrum. Problems with M-ary modulation schemes include increased equipment complexity for the transmitter and receiver, and higher error rates because the decision regions between symbols, such as phase angles or amplitude levels, for example, become closer. Using M-ary signaling schemes, accordingly, can cause degradation in channel performance. Thus, a need exists for a system and method that provides a high capacity spread-spectrum channel, which does not have increased equipment complexity, a degradation in performance, or loss of the advantages of spread-spectrum modulation.
Today, high data rate transmission employs 64QAM without spread spectrum. These systems are extremely sensitive to noise since the separation of symbols is small. They therefore require a high signal-to-noise ratio. Hence, all interference must be small. This limits their capability to efficiently use the spectrum.
A general object of the invention is a synchronous, high-capacity, spread-spectrum-communications system.
Another object of the invention is a system and method for communicating information at a high rate through a spread-spectrum channel, while maintaining the advantages of spread-spectrum including high processing gain, resistance to fading and an ability to share the same spectrum with other users.
A further object of the invention is to synchronously demodulate a plurality of modulated-data signals embedded in a spread-spectrum-communications signal.
According to the present invention, as embodied and broadly described herein, a high capacity spread spectrum communications system for use over a communications channel is provided comprising a high-capacity-spread-spectrum transmitter and a high-capacity-spread-spectrum receiver. The high-capacity-spread-spectrum transmitter includes demultiplexer means, generic means, a plurality of message means, summer means, and transmitter means. The demultiplexer means demultiplexes the message data into a plurality of demultiplexed-data signals. The generic means generates a generic-chip-code signal. The plurality of message means generates a plurality of message-chip-code signals. Each of the plurality of demultiplexed-data signals and each of the plurality of message-chip-code signals are synchronized to each other and optionally to the generic-chip-code signal. The message-chip-code signals must be orthogonal or near orthogonal for the high-capacity-spread-spectrum system to work.
Each spreading means of the plurality of spreading means, spread-spectrum processes a demultiplexed-data signal with a respective message-chip-code signal to generate a spread-spectrum-processed signal. The summer means combines the plurality of spread-spectrum-processed signals, and if used, the generic-chip-code signal. The combined signal typically is a multi-level signal, with an instantaneous-combined voltage level equal to the sum of the voltage levels of the plurality of message-chip-code signals and, if used, the generic-chip-code signal. A multi-level signal is a signal with multiple voltage levels.
The transmitter means transmits the combined plurality of spread-spectrum-processed signals, and if used, the generic-chip-code signal, on a carrier signal over the communications channel as a spread-spectrum-communications signal. While the transmitter means may use a linear power amplifier for optimum performance, a nonlinear power amplifier also may be used without significant degradation or loss in performance. Thus, the sum of voltage levels need not be an exact linear sum, with only a little loss in performance.
The high-capacity-spread-spectrum receiver can be used for simultaneously receiving a plurality of spread-spectrum channels of a spread-spectrum-communications signal. The high-capacity-spread-spectrum receiver includes acquisition and tracking means, multiplexer means, and a plurality of spread-spectrum receivers, and optionally generic-spread-spectrum-processing means. Each spread-spectrum receiver includes message-spread-spectrum-processing means, detection means and bit-synchronization means.
The generic-spread-spectrum-processing means, if used, recovers a carrier signal from a spread-spectrum channel of a received spread-spectrum-communications signal, and generates a replica of the generic-chip-code signal. The acquisition and tracking means acquires and tracks the recovered carrier signal. The acquisition and tracking means also synchronizes the generic-spread-spectrum-processing means to the recovered carrier signal.
The generic-spread-spectrum-processing means is not required for the high-capacity-spread-spectrum receiver. The acquisition and tracking means may be coupled to an output of a message-bandpass filter, and appropriate circuitry, such as that shown in FIG. 1, can be used to recover the carrier signal.
The plurality of message-spread-spectrum-processing means despreads the received spread-spectrum-communications signal as a plurality of modulated-data signals. The plurality of message-spread-spectrum-processing means derives synchronization from one of the modulated-data signals, or a replica of the generic-chip-code signal provided by generic-spread-spectrum-processing means.
The plurality of detection means detects the plurality of modulated-data signals as a plurality of detected signals, respectively. The plurality of detection means may be nonsynchronous or synchronous, for converting the plurality of modulated-data signals to the plurality of detected signals, respectively.
The bit-synchronization means uses one of the modulated data signals or the replica of the generic-chip-code signal produced by the generic-spread-spectrum-processing means for synchronizing the xe2x80x9cintegrating and dumpingxe2x80x9d of the detected signal. The xe2x80x9cintegrated and dumpedxe2x80x9d detected signals are referred to as demodulated signals. The multiplexer means multiplexes the plurality of demodulated signals from the plurality of bit-synchronization means as received-message data.
A second embodiment of the high-capacity-spread-spectrum receiver for simultaneously receiving a plurality of spread-spectrum channels includes acquisition and tracking means, multiplexer means, and a plurality of spread-spectrum receivers, demodulation means, switching means, and optionally generic-spread-spectrum-processing means. Each of the plurality of spread-spectrum receivers has message-spread-spectrum-processing means. The generic-spread-spectrum-processing means, if used, generates a replica of the generic-chip-code signal. The generic-spread-spectrum-processing means uses the replica of the generic-chip-code signal for recovering a carrier signal from a spread-spectrum channel of the received spread-spectrum-communications signal. The acquisition and tracking means acquires and tracks the recovered carrier signal, and synchronizes the generic-spread-spectrum-processing means to the recovered carrier signal. Alternatively, a demodulated data signal from the spread-spectrum-communications signal can serve for synchronization.
Each of the message-spread-spectrum-processing means despreads the respective spread-spectrum channel of the received spread-spectrum-communications signal as a modulated-data signal. Each of the message-spread-spectrum-processing means derives synchronization from a replica of the generic-chip-code signal provided by the generic-spread-spectrum-processing means, or one of the demodulated-data signals from the received spread-spectrum-communications signal.
A single demodulating means is employed for demodulating each modulated-data signal as a respective demodulated signal. The demodulation means includes detection means and bit-synchronization means. The demodulation means is used on a time-shared basis. Accordingly, the detection means sequentially detects each of the plurality of modulated-data signals from the plurality of message-spread-spectrum-processing means, as a detected signal. The detection means may be nonsynchronous or synchronous, for converting each of the plurality of modulated-data signals to a detected signal. Each of the detected signals is xe2x80x9cintegrated and dumpedxe2x80x9d by bit-synchronization means. The bit-synchronization means derives synchronization from a replica of the generic-chip-code signal produced by generic-spread-spectrum-processing means.
Switching means is coupled between an input of the demodulation means and each output of the message-spread-spectrum-processing means. The switching means also is coupled between the output of the demodulation means and a plurality of inputs of the multiplexer means. The switching means switches the demodulation means between each of the message-spread-spectrum-processing means and each input of the multiplexer means, respectively. A single demodulation means accordingly demodulates, by time sharing, each of the modulated-data signals as a respective demodulated signal, from each of the message-spread-spectrum-processing means. The multiplexer means, by time-sharing the demodulation means, combines each of the demodulated signals from the demodulation means to generate the received-message data.
The present invention also includes a method for synchronously modulating and demodulating spread spectrum communications. The method comprises the steps of generating a generic-chip-code signal and a plurality of message-chip-code signals. The message data are demultiplexed into a plurality of demultiplexed-data signals. Each of the plurality of demultiplexed-data signals are modulo-2 added to a message-chip-code signal to generate a spread-spectrum-processed signal. The generic-chip-code signal and the plurality of spread-spectrum-processed signals are combined and transmitted on a carrier signal over the communications channel as a spread-spectrum-communications signal.
At the receiver, the steps include recovering the carrier signal from a received spread-spectrum-communications signal and despreading the received spread-spectrum communications signal as a plurality of modulated-data signals. The recovered-carrier signal is used to synchronize the step of generating a replica of the generic-chip-code signal, or other synchronization signal. More particularly, a replica of the generic-chip-code signal is correlated with the received spread-spectrum-communications signal, which has a generic channel defined by the generic-chip-code signal at the transmitter. If the signal out of the generic-bandpass filter is small, then the acquisition and tracking circuit delays the phase of the generic-chip-code signal and the correlation process is repeated. If the phase of the replica of the generic-chip-code signal and the generic-chip-code signal in the spread-spectrum-communications signal are the same, then the output of the generic-bandpass filter will be at a high voltage level.
A plurality of replicas of the message-chip-code signals is synchronized to the replica of the generic-chip-code signal for despreading the received spread-spectrum-communications signals as a plurality of modulated-data signals. The plurality of modulated-data signals is detected as a plurality of detected signals. The recovered-carrier signal optionally may be used to synchronously demodulate the plurality of modulated-data signals as the plurality of detected signals. Each of the detected signals is synchronously converted to a demodulated signal, by using timing from the replica of the generic-chip-code signal to control xe2x80x9cintegrating and dumpingxe2x80x9d functions of a lowpass filter and electronic switch. The plurality of demodulated signals is multiplexed to generate the received message-data signal.